18.
The Myth of
“Sustainable
Yield”
• “Sustainable yield” as a percent of recharge is an old but persistent
misconception
– Still provides a useful “rule of thumb”…but it’s not the real determinant
• Thiess first pointed out the error in 1940
– Even today, even many hydrologists still don’t get it
– Recent papers by prominent hydrologists notwithstanding….
• One use of models is to help evaluate how pumping changes the system
– In terms of both quantity and quality
• But models cannot replace the need for observed data
– In fact, they require data—lots of it!
• Proper management
– Comes from observing the trends and responding according
– Requires effective inter-agency cooperation and decision-making
- Which requires an inter-agency framework for ongoing consultation
• Finally, it addresses only the supply question; overlooks the demand
aspect of water management

19.
Economics of
Sustainability (101)
• Higher salinity is the natural
consequence of production
• Green* technologies seem at
first to be the obvious choice
– But they may be (and usually
are) more expensive…
- Unless the incremental cost
is lower in the long run…
• The affordable limit may change
– Upward if the economy is
growing…
• Regulatory limits should be
routinely re-examined
– And adjusted based on
observable or predictable
consequences…
• “Sustainable” really means
“economical”
– i.e., meeting the objective with
least expenditure of resources Production
Salinity regulatory limitCost
affordable limit
*Green = satisfies a particular environmental
objective that is widely acknowledged as desirable
vertical wells
tunnel wells
High start-up, high ops cost
desalination
Low and high
switch places

20.
Sustainable
Use
• The old question….
– “What is the sustainable yield…?”
• Is the wrong question!
• The right question…?
– “How can we economically meet the potable water needs of the
entire community?”
- Demand—just as important as supply
– Pricing structure
– Conservation incentives
- Supply—how much of a given quality at a given cost?
– Alternative technologies…and their costs
– Modeling can help—if the model is accurate
– Is only as good as the data
– The right kinds of data, and enough of them
– Sufficiently accurate and precise to do the job….